A dual-task tool for quantifying normal comprehension of aphasic connected speech production: A constructive replication

Background: Deficits of attention or its control secondary to brain damage have been proposed as all or part of the underlying mechanisms for the linguistic impairments that characterise aphasia (Clark & Robin, 1995; Granier, Robin, Shapiro, Peach, & Zimba, 2000; McNeil, 1982, 1988; McNeil, Odell, & Tseng, 1991; Murray, Holland, & Beeson, 1997a; Tseng, McNeil, & Milenkovic, 1993). McNeil, Doyle, Hula, Rubinsky, Fossett, and Matthews (2004) developed a set of tasks to quantify the difficulty that normal listeners have in understanding the language production of persons with varying amounts of aphasia. In their dual-task study, a significant decrement was found in the visual–manual tracking accuracy of normal older individuals while concurrently listening to the connected language of a person with moderate, as compared to mild, aphasia. No performance costs were observed on the listening tasks across three tracking difficulty levels. Possible reasons for the unidirectional performance cost were speculated and the present study was designed to investigate one of them.Aims: Using the same story comprehension task used in the previous study and the same visual–manual tracking task, but increased in difficulty, this study sought to investigate whether the increased demands of the tracking task were sufficient to elicit a concurrent cost on story comprehension performance.Methods & Procedures: A total of 24 normal participants performed the tracking and story comprehension tasks concurrently and in isolation. Story retell performance was evaluated within subjects across two tracking difficulty levels (easy and hard) and tracking performance was evaluated between subjects across three story difficulty levels (no story, mild difficulty, and moderate difficulty).Outcomes & Results: Tracking performance varied significantly across story task difficulty in the easy tracking condition, with participants demonstrating better tracking performance in the mild story condition than in either the moderate story or no story conditions. None of the comparisons made with the harder tracking condition reached significance. There was no effect of tracking difficulty on story comprehension as measured by subsequent story retell performance.Conclusions: The findings from this study replicate the findings from the McNeil et al. (2004) study that these dual-tasks show a reliable cost of story difficulty on concurrent tracking performance. Contrary to predictions, no effect of tracking difficulty on story retell performance was found, despite the increased tracking difficulty used compared to the previous study. While this finding does diminish the probability that insufficient tracking task difficulty was the source of the unidirectional costs in the previous study, it leaves a number of alternative explanations for the findings viable and unaddressed.